The effects of DNA-reactive drugs on DNA in mammalian cells is being characterized and related to cytotoxic mechanisms. Alkaline elution and related filter methods are being utilized to measure DNA single-strand breaks, DNA double-strand breaks, DNA interstrand crosslinks, DNA-protein crosslinks, and alkali-labile sites. The DNA intercalating agents, m-AMSA and adriamycin, produced protein-associated DNA single-strand and double-strand breaks, probably by a topoisomerase mechanism. The DNA effects of m-AMSA, but not adriamycin, were rapidly reversible. The fluoro analog of CCNU produced DNA crosslinking effects and differential cytotoxicity dependent on meG repair capacity in human cells. The effects were similar to those of CCNU except that interstrand crosslinking was much more delayed. The effect of carbamoylation on differential cytotoxicity was studied in hydroxy-CCNU derivatives. Resistance to cis-Pt(II) in a line of L1210 cells was attributed to quenching of Pt-DNA monoadducts, probably due to increased cell thiol content, thereby preventing DNA crosslink formation.